FA head power curve?

The FA head intake venturi is approx 32mm wide and 26mm tall which is a bit small. Good velocity and better midrange power. But I heard a report that they are not that great on top end - possibly less top end power. I have not tested one myself so I don't know and that's why I'm asking. Has any one made an objective comparison to a stock head such as:

750 30mm ports

850, RH4 32mm 32mm ports RH10 30mm ports

And all the negative talk about 32mm RH4 heads - with a dyno comparison - are they worse or better performance wise than the 30mm port RH10 head. Note that the screaming Combat had a 32mm port head. And there are a lot of Maney heads with ports larger than 32mm that have been winning races. There has also been tunning notes port specs and HP gain claims with 32mm ports.

I have always started with 28mm port heads and raised the ports while refering to Axtell port dimensions with great results, but have stayed away from the 32mm port heads which have a rep of being too big (even though my 850 race ported head was pushing 31 or 32mm at the venturi and 34 at the manifold.

There has been lots of talk but I haven't seen an objective dyno test.

Why are you seeking more top end when the rev limit is 7000 RPM due to the bottom end ? I suspect the low balance factor of the Commando crank, might stop the motor from developing top end, even without rider restraint. When you rev a crank high when it is not suitably balanced, energy is transferred to its surrounds by vibration.
It is probably more important to make the Commando engine pull harder as it spins-up, rather than develop more top end. Unless you believe more top end must mean there is more mid-range ?
The gearbox is a torque converter. More gears, closer with high overall gearing, might give better results than playing with ports.

jseng1 said:

The FA head intake venturi is approx 32mm wide and 26mm tall which is a bit small. Good velocity and better midrange power. But I heard a report that they are not that great on top end - possibly less top end power. I have not tested one myself so I don't know and that's why I'm asking. Has any one made an objective comparison to a stock head such as:

750 30mm ports

850, RH4 32mm 32mm ports RH10 30mm ports

And all the negative talk about 32mm RH4 heads - with a dyno comparison - are they worse or better performance wise than the 30mm port RH10 head. Note that the screaming Combat had a 32mm port head. And there are a lot of Maney heads with ports larger than 32mm that have been winning races. There has also been tunning notes port specs and HP gain claims with 32mm ports.

I have always started with 28mm port heads and raised the ports while refering to Axtell port dimensions with great results, but have stayed away from the 32mm port heads which have a rep of being too big (even though my 850 race ported head was pushing 31 or 32mm at the venturi and 34 at the manifold.

There has been lots of talk but I haven't seen an objective dyno test.

Click to expand...

I use an RH10 head on my 850. It has 30mm ports, with 34mm Mk2 Amal carbs. The taper on the needles compensates for loss of vacuum. I use 6D Mikuni needles which are the slowest taper. Even if I grab a very quick handful of throttle, I never get a gasp. So 32mm ports might still be OK
If the ports are too big and you do not have enough taper on the needles, you usually get a symptom.
I think most guys would be using quicker taper needles. than those which I use.

The problem is that, if you are jetted even slightly too rich, the throttle response is slower. A dyno might not show that.

jseng1 said:

The FA head intake venturi is approx 32mm wide and 26mm tall which is a bit small. Good velocity and better midrange power. But I heard a report that they are not that great on top end - possibly less top end power. I have not tested one myself so I don't know and that's why I'm asking. Has any one made an objective comparison to a stock head such as:

750 30mm ports

850, RH4 32mm 32mm ports RH10 30mm ports

And all the negative talk about 32mm RH4 heads - with a dyno comparison - are they worse or better performance wise than the 30mm port RH10 head. Note that the screaming Combat had a 32mm port head. And there are a lot of Maney heads with ports larger than 32mm that have been winning races. There has also been tunning notes port specs and HP gain claims with 32mm ports.

I have always started with 28mm port heads and raised the ports while refering to Axtell port dimensions with great results, but have stayed away from the 32mm port heads which have a rep of being too big (even though my 850 race ported head was pushing 31 or 32mm at the venturi and 34 at the manifold.

There has been lots of talk but I haven't seen an objective dyno test.

Click to expand...

Re the Rh4 vs rh10 question
Here's a 2018 post from Jim Comstock.
"I tested both heads on the same stock bottom end several years ago. I used the same 32mm carbs.
The maximum horsepower was basically the same.
The rh10 made more power below 5000 than the RH4. As I recall the largest difference was 3 or 4 horse at around 4000 rpm.
I also tried 32mm straight manifolds on the rh10 and tapered the ports in the RH10 to match. It then made less power than either stock head. Jim"

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Its not surprising that the small ports would have better midrange.
It is surprising that the large ports don't make extra power at the top.
This might be due to the stock inlet valve size being the power limiting factor.
It's quite likely that with a bit larger inlet valve fitted to both heads the RH4 could produce the highest peak horsepower

Glen

Yes, at that size the port is most likely no longer the bottleneck, something else is. Therefore there’s more to be had by finding and addressing said bottleneck.

My belief is there’s probably nothing to gain by enlarging the ports on an otherwise stock Commando. But that that changes as the rest of the ‘package’ changes.

The Combat does seem to like it’s 32mm ports, but remember it has a 2S cam and 10:1 CR too.

Comnoz didn’t enlarge my RH10 ports on my hopped up 850 and the before and after Dyno runs of that head work was + 9 bhp!

But that said, my current motor has 36mm ports, as does the one currently ‘in build’.

All only IMHO of course.

Flow is a function of pressure and velocity. The pulse in a smaller port is probably at a higher pressure. The valve size is probably designed to cope with flow in a small port. With a big port, the rate of flow has to increase at the valve, if the valve area is not big enough. But that must happen anyway.
There can be a noticeable improvement in performance, if the valve seat is cut with three angles, instead of just one angle.
There are father and son who were racing pushrod Matchless single in historic races. I looked down the carb one day - the inlet port was huge. I suggested that they should consider racing a Jawa speedway motor, and they would start where they finished with the Matchless motor. So they bought two motors from Neil Street who used to spend the Australian winters with the Poole Speedway riders in the UK. First time out, they beat the quick guy on the Molnar Manx. In their history, the inlet port of Jawa two valve motors was decreased in size.

I don't have any problem with top end power. If I let it, my motor's revs would go straight through the ceiling, - and my gearing is absurdly high. As you wind the throttle on, the taper on the needles usually increase the rate of fuel flow into the inlet tracts. The difference between good power and excellent in terms of fuel flow is extremely small. I suggest you should try slower taper needles. If your needle jet size is correct and you can ride the bike without getting the cough ,, you should be faster. The slightest bit too rich, will take the edge of the motor.
The 850 motor is like the featherbed frame, it is difficult to imagine how Norton got it so right.

worntorn said:

Its not surprising that the small ports would have better midrange.
It is surprising that the large ports don't make extra power at the top.
This might be due to the stock inlet valve size being the power limiting factor.
It's quite likely that with a bit larger inlet valve fitted to both heads the RH4 could produce the highest peak horsepower

Glen

Click to expand...

I am not surprised that larger ports don't make more power at the top. - Bullshit baffles brains. 'Because I can' is never a good reason for doing anything.
My short stroke 500cc Triumph motor was a modified 650. I ported the head even bigger and made it's performance better - if it revved over 10,000 RPM. But when it was able to rev that far, the bike became impossible to ride - it had nothing much anywhere else. I fitted a 2 into 1 pipe and I started to get decent lap times. But without that, it was deadly.
It might be that you don't rev a Commando motor high enough to see an improvement from a bigger port ? With a 650 Triumph, porting a Bonneville head can make them slower.

I'm not sure if this will add anything of meaning to this thread but I have an 850 (cNw) with a FullAuto head and twin FCR 35s, and it it very tractable at lower revs but I have taken it to 128mph at about 7300rpm. That's all it's got. I think it's a 21 tooth front (much work to confirm).
@cNw may be able to confirm gearing (Hi Matt!).
I'm happy with that.
Cheers

robs ss said:

I think it's a 21 tooth front (much work to confirm).

Click to expand...

Not really, you can paint one tooth and count as you revolve the rear wheel.

Pulling 7300 in top sounds healthy to me.

Hi Rob…….you have a 22 tooth

Matt

cNw said:

Hi Rob…….you have a 22 tooth

Matt

Click to expand...

That was easier than lying on the floor and painting a sprocket tooth !

Here is a dyno chart for an 850 with Fullauto vs a " late 750" stocker.
The " late model " 750 does very well, even shows a bit more low end and midrange than the 850.
Experimental cam in the 850.
Perhaps Madnorton will comment.

worntorn said:

Here is a dyno chart for an 850 with Fullauto vs a " late 750" stocker.
The " late model " 750 does very well, even shows a bit more low end and midrange than the 850.
Experimental cam in the 850.
Perhaps Madnorton will comment.

Burlen Ltd The World Of Burlen

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amalcarb.co.uk

Click to expand...

The 750 may do so well as it has better fuelling, the 850 appears to be a little rich.

acotrel said:

I don't have any problem with top end power. If I let it, my motor's revs would go straight through the ceiling, - and my gearing is absurdly high. As you wind the throttle on, the taper on the needles usually increase the rate of fuel flow into the inlet tracts. The difference between good power and excellent in terms of fuel flow is extremely small. I suggest you should try slower taper needles. If your needle jet size is correct and you can ride the bike without getting the cough ,, you should be faster. The slightest bit too rich, will take the edge of the motor.
The 850 motor is like the featherbed frame, it is difficult to imagine how Norton got it so right.

Click to expand...

Acotrel, I'm surprised you didn't get snapped up by the John Player race team- but hang on a minute , they were restricted to pump fuel, whereas you are running your bike on some high octane methanol fuel

worntorn said:

Re the Rh4 vs rh10 question
Here's a 2018 post from Jim Comstock.
"I tested both heads on the same stock bottom end several years ago. I used the same 32mm carbs.
The maximum horsepower was basically the same.
The rh10 made more power below 5000 than the RH4. As I recall the largest difference was 3 or 4 horse at around 4000 rpm.
I also tried 32mm straight manifolds on the rh10 and tapered the ports in the RH10 to match. It then made less power than either stock head. Jim"

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Click to expand...

FA tapered port as viewed from the side. The raised floor should have advantages - at least for the midrange.

worntorn said:

Here is a dyno chart for an 850 with Fullauto vs a " late 750" stocker.
The " late model " 750 does very well, even shows a bit more low end and midrange than the 850.
Experimental cam in the 850.
Perhaps Madnorton will comment.

Burlen Ltd The World Of Burlen

Our articles are sorted chronologically. Over the past 12 years we have added x articles giving advice, reviews and thoughts about [page_title]

amalcarb.co.uk

Click to expand...

Yes I can, the FA head is one of Ken's and prior to an update that was applied to the later heads Ken produced, I think the STS version are different again. The cam I have fitted is one Peter Williams designed prior to his passing, it has 10.5mm of lift on the cam and the LSA is quite wide, some said it would not work. Carbs were 932's premiers with the needle on the mid setting, now on the top setting to weaken the mixture. Exhausts are straight through peashooters and air filter was standard black box., pistons are standard GPM. The torque curve from the 850 is nice and flat which makes for a nice ride. I discussed the results with Steve Maney and he was surprised it delivered as much as it did in the configuration I had used as stock 850's produce a lot less than what was claimed from the factory. He suggested to get more from the fat 850 a valve job would help as this would allow more revs prior to it 'rolling over' as he considered that the inlet valve is not large enough on the 850 and that is why they rev out lower than the 750.

We hope to build a 750 engine in the spring with off the shelf parts we have here, as we are intrigued to see what the like of the Omega's, PW3 cam and FA head produce. This engine will be dyno'd.

There seem to be many engines built but very few seem to make it to the dyno which is a shame as it doesn't cost much for a dyno run.

if anyone fancies trying my bike out them they are more than welcome, it needs a ride as my arm doesn't seem to be getting better that quick which is preventing me from riding at the moment.

I suspect that a hot cam in an 850 with stock inlet valve size is a bit of a net negative. It reduces that extra 850 low and midrange grunt to give more power at top, but the inlet valves won't allow the cam to do it's thing at the top.
This dyno chart appears to show that.



Glen

worntorn said:

I suspect that a hot cam in an 850 with stock inlet valve size is a bit of a net negative. It reduces that extra 850 low and midrange grunt to give more power at top, but the inlet valves won't allow the cam to do it's thing at the top.
This dyno chart appears to show that.



Glen

Click to expand...

That’s my hypothesis too. Same with regards to fitting a bigger cam and keeping stock CR, the cam costs some bottom end but then the CR doesn’t let it work as intended higher up either. Lose / lose.

That was my point about it being the bottleneck earlier.